Yesterday, Bo cantered.
What? A horse cantering is an achievement?
It is if the horse concerned is barely trained and this is the first time anyone other than his original trainer has managed to convince him that cantering under saddle is actually possible.
Achievements are, well, relative. Your first credit is an achievement, but selling another story to the same magazine? Less of one. I'd go so far as to say that for something to feel like an achievement is has to register as an improvement. In writing, this might mean selling to an editor who rejected you 100 times with other works or selling for a higher rate.
Finishing your first novel, thus, is a greater achievement than finishing your second one - at least in your psychology.
But then, you can look at this another way. Everything you finish is an achievement. If you see it that way, you will feel better. Trust me...it's hard to see finishing a novel as an achievement if you haven't managed to sell the last one, but it is.
And if that doesn't help, remember that there are people out there for whom getting through the day is an achievement.